How Hot Is Your (Compost) Heap?

Q. What is the point of taking a compost pile’s temperature? What temperature should it be?

Cathy Munoz
Naperville, Illinois

A. When ingredients going into the compost pile include diseased or pest-infested plant matter and/or weed seeds, hot composting offers a way to deal with those problem organisms so they don’t return to your garden in the finished compost. Frequent aeration—achieved by turning the pile—and temperature monitoring are necessary to ensure that hot, or thermal, composting controls bad actors like weeds and diseases without producing so much heat that the compost bursts into flame.

Elaine Ingham, Ph.D., chief scientist at the Rodale Institute, compares a thermal compost pile to a lively party scene: “Just think about any crowded bar, with people dancing, partying, and getting rowdy, and think of the heat that can be generated in a place like that,” Ingham says. That’s what happens in a compost pile where nitrogen-containing foods such as legumes, seed meals, and manures get the party started by boosting the activity of the decomposers in the pile. As bacteria and fungi grow and reproduce rapidly under the influence of high-nitrogen organic materials, heat is released.

To generate the heat needed to thoroughly process problem ingredients, Ingham notes, the party in your compost pile has to go for 10 to 15 days. This takes quite a bit of food, and the pile needs to be turned at least five times during those 2 weeks to get oxygen back into the pile and to cool things off a bit. “Also, the pile needs to be evened out, so the really hot spots get mixed with the ‘wallflowers,’” she continues.

After a compost pile is constructed, rising temperatures within the pile indicate that microbes have begun to break down the organic materials. Regular temperature monitoring lets you make sure the decomposers are at work long enough to kill diseases, pests, parasites, weed seeds, and pest nematodes. While experienced compost chefs may be able to judge their pile’s temperature by feel, a 20-inch compost thermometer is a worthwhile investment of about $30 to provide a more reliable measurement of temperatures within a heap. Composting fanatics might opt to splurge on a thermometer with a 3-foot probe, costing $120 or more and able to take readings from deeper within a pile. Using the thermometer’s long probe, monitor the compost daily during the first week to 10 days to see if the interior of the pile remains in the desirable range of 140°F to 160°F.

Your compost pile can get too hot, Ingham cautions. “If the pile starts getting into the range of 160 to 165 degrees, the organisms are growing so fast that they can use up the oxygen in the pile, causing the good-guy aerobic organisms to start to go dormant or die.” Then the anaerobic bacteria and yeasts (the anaerobic forms of fungi) start growing and, sooner or later, alcohol will be produced. “There are hundreds of different kinds of alcohols that these anaerobic organisms can make, and some have a heat of combustion around 180°F,” warns Ingham. “If the pile is anaerobic and temperatures reach 180°F or thereabouts, it is very likely that the pile will spontaneously combust.” To prevent this from happening, turn the pile if its temperature exceeds 165 degrees F.

Conversely, a pile that is short on nitrogen-rich ingredients may never reach the thermal range needed to wipe out pathogens and weedy seeds. If things aren’t heating up, it’s time to mix in some high-nitrogen materials, such as fresh grass clippings, legumes, cottonseed meal, or manure, as you moisten and aerate the pile.

After your first attempt at thermal composting, Ingham says, you figure out how much high-nitrogen “party” food, green bacterial food, and woody, high-carbon food to add to make things heat up. “Monitoring temperature takes mere minutes,” Ingham concludes, “but is essential to letting you know when you need to turn the pile or add party foods to keep things just right for your guests.”